The most common site for extra-intestinal Escherichia coli infection is the urinary tract. There are a number of risk groups for this site of infection, one of which is young women in their sexually active years. Infection in this group accounts for 6-7 million office visits annually. Escherichia coli isolates are responsible for greater than 80-90% of these episodes. About 250,000 of these infections progress to acute pyelonephritis. In approximately 20% of these women frequent recurrences of infection (3-6/annum) follow the first episode. The morbidity, time lost from work and cost of this infectious syndrome is considerable. An improved understanding of the pathogenesis of this disease would be of significant importance. It is believed that the vast majority of recurrent UTIs are episodes of reinfection with different strains of E. coli rather than persistence of a single strain. We are employing pulsed field gel electrophoresis to generate DNA "fingerprints" of bacterial strains isolated from women with recurrent UTIs. This will allow us to definitively establish how frequently a single versus different strains are responsible for this pattern of infection. Our studies to date have examined isolates from 23 women with recurrent urinary tract infections. The majority of symptomatic episodes in a given women are due to the same isolate. To assess whether the responsible strain was persisting within the gastro-intestinal tract or within the urinary tract, faecal isolates from 6 women were examined. This evaluation is consistent with the hypothesis that persistence within the gastro-intestinal tract is the mechanism by which the same isolate is responsible for recurrent UTIs in a given woman. These findings have significant implications for the treatment of women with recurrent UTIs.